New Technique Maps Dark Excitons with Precision

According to a recent study published in the journal Nature Photonics , an international team of researchers  led by the University of Göttingen  has introduced a new technique for ultrafast imaging of dark excitons.   

How can we further improve emerging technologies like solar cells? The researchers used a groundbreaking technique to investigate this question. For the first time, they precisely tracked the formation of elusive particles called dark excitons, which could play a key role in the future of solar cells, LEDs and detectors.

Dark excitons are tiny pairs of electrons and holes that are left behind when excited. They carry energy but do not emit light, hence the name “dark.” Excitons can be visualized as a balloon (representing an electron) floating away, leaving behind an empty space (a hole) that remains bound by a force called the Coulomb interaction.

The researchers describe this as a “particle state” that is difficult to detect but crucial in atomically thin, two-dimensional structures within special semiconductor compounds.

In previous work, the research group of Professor Stefan Matthias at the Department of Physics at the University of Göttingen demonstrated how dark excitons form so rapidly and explained their dynamics using quantum mechanical theory.

In their latest study, the team developed and applied for the first time a new technique called “ultrafast dark-field momentum microscopy.” This breakthrough enabled the researchers to observe the formation of dark excitons in tungsten diselenide (WSe₂) and molybdenum disulfide (MoS₂) materials, which occurs over the course of just 55 femtoseconds (0.000000000000055 seconds), with a resolution precise to 480 nm (0.00000048 m).

This method allows us to measure the dynamics of charged particles very precisely. The results provide fundamental insight into how the sample properties affect the motion of charged particles, which means that this technique could potentially be used in future to specifically improve the quality and therefore performance of, for example, solar cells . 

Dr. David Schmidt, lead author of the study, from the Department of Physics at the University of Göttingen

 “This means that the technique can be used not only in these specially designed systems but also to study new types of materials, ” added Dr. Marcel Reutzel, Junior Research Group Leader in Matthias’ research group. 

The research was funded by the DFG-funded Collaborative Research Centres “Control of Energy Conversion at the Atomic Scale” and “Experimental Mathematics” in Göttingen and “Structure and Dynamics of Internal Interfaces” in Marburg.